The 2007-born protege sustained the problem deep into injury time in Barcelona's 2-1 La Liga win over Rayo Vallecano on August 27, and he was in tears as the medical staff helped him off the field.
As well as rupturing his ACL, which carried a significant layoff in itself, Bernal was also revealed to have hurt his meniscus, adding to the teenager's lengthy rehabilitation time.
The midfielder required an immediate operation, and Barcelona confirmed on Monday that the procedure had been successful, but Bernal would likely be recovering until September 2025.
"The player Marc Bernal has undergone a successful surgical procedure on the injury to the anterior cruciate ligament and lateral meniscus in his left knee carried out by Dr. Joan Carles Monllau under the supervision of the club's medical services at Barcelona Hospital," a club statement read. "Recovery time is expected to be 12 months."
How big a blow is Bernal's 12-month layoff?
Barcelona supporters have now been there and done it all when it comes to their top teenage talents picking up cruel and debilitating injuries, and Bernal's comes at a time when he was establishing himself in Hansi Flick's setup.
Thanks to other injuries elsewhere in the squad, the teenager was given the chance to lay down a marker in pre-season and made his opportunities count, thereby forcing his way into Flick's starting XI for the new season.
Bernal had started each of Barcelona's opening three La Liga games of the season - all three of which La Blaugrana won - before suffering his crushing blow, helping to see out 2-1 victories over Athletic Bilbao, Valencia and Vallecano.
Right before the start of the season, the 6ft 3in midfielder signed a new Barcelona contract until 2026, and those terms can be extended by a further three years until 2029 when he turns 18 next May.
Bernal came through Barcelona's famous La Masia academy setup and has also won nine caps for Spain's Under-17 side, making four appearances at the 2023 World Cup as his nation reached the quarter-finals.
Which other Barcelona players are out injured?
The Barcelona doctors are looking after a host of stricken players at present, and Bernal is just one of four midfielders in the Blaugrana infirmary, which also houses long-term victims Gavi (ACL) and Frenkie de Jong (ankle).
Twenty-one-year-old Fermin Lopez has joined them in the medical bay with a hamstring injury, which he picked up while representing Spain's Under-21s, and he is believed to be facing a three-week layoff.
Danish defender Andreas Christensen - who has sometimes filled in as a number six for Barcelona in recent months - is also expected to be out until 2025 with an Achilles problem, while Ronald Araujo's hamstring issue could sideline him until October.
However, Flick has been handed one piece of encouraging injury news, as Mundo Deportivo reports that attacker Ansu Fati could overcome a foot problem in time for Sunday's trip to Girona.
Barcelona sit four points clear of Real Madrid, Atletico Madrid and Villarreal at the top of the La Liga table, and Flick's men are the only side to have won their opening four games of the season.